Field of the Invention
This invention relates to a system for controlling the slip of a clutch transferring rotational force from a power source to an output shaft, and more particularly, to controlling the clutch pressure and clutch slip speed to reduce torsional vibrations generated by the entire powertrain.
Background of the Invention
The torque converter clutch is a fluid operated friction device engageable to couple an input shaft to an output shaft via a clutch. Typically the clutch is either fully released to permit unrestrained slippage between the input shaft and the output shaft, or fully engaged, also referred to as “locked-up,” to prevent such slippage entirely. An unfortunate aspect of full clutch engagement is that the engine and any subsequent moving parts produce torsional vibrations, normally absorbed by the torque converter, that are passed directly through the clutch to the remainder of the powertrain. Torsional vibrations are a product of the mass and geometry of the entire system. Output and input shaft length, firing order of the engine, mass of the rotating assembly, and related parts, all contribute to the overall mass and geometry. Due to this mass and geometry, the system has frequencies at which it will naturally resonate. These resonant frequencies produce torsional vibrations when the system is operated under certain loads. These torsional vibrations produce damaging pulsations therein if not properly dampened that can significantly reduce the life of the power train components. Additionally, each unique system may include unique frequencies at which torsional vibrations are produced. Different loads, different rotating assemblies, and different equipment will all produce torsional vibrations at their own respective frequency. As a result, it is desirable to dampen these torsional vibrations as they are known to physically vibrate the system with such force that, not only can the clutch be damaged, but the entire drive line may be damaged as well.
In addition to the above-mentioned components influencing torsional vibrations, other devices may contribute as well. For example, in hydraulic fracking, a pump is used to pump hydraulic fluid deep into the ground. Both the engine and the pump produce vibratory pulsations during the power stroke which manifest as torsional vibrations in the system.
Vibration absorbing couplings have been used to absorb these types of vibrations. While the couplings are effective, they come at a significant cost and add considerable size to the torque converter and clutch mechanism. As a result, it is optimal to reduce or eliminate torsional vibrations without any added components, but with software control of clutch pressure and clutch slippage.
As a result, it has been proposed to operate the clutch in a slipping mode, wherein a predetermined amount of slippage between the torque converter and clutch is permitted. In such a system, the objective is to isolate engine torque perturbations in the torque converter, while passing steady state engine torque at a slip rate that provides improved torque converter efficiency and extends component life.
A typical clutch transfers rotational force through a coefficient of friction applied against the torque converter. This transfer is nonlinear in nature, and the potential for instability is present at various slip speeds. Characteristically, the fluid pressure required to maintain a given level of slippage tends to decrease as the slippage increases. As a result, there is a tendency for the slippage control to completely engage the clutch with maximum pressure in response to a condition for which the measured slip exceeds the desired slip.
One common application where torsional vibrations are experienced and require dampening is found in the hydraulic fracturing industry, which uses a hydraulic torque converter with a lock-up clutch. In hydraulic fracturing applications, in order to achieve maximum efficiency, the lock-up clutch is preferably fully engaged 100% of the time a fracking pump is in operation. Because the lock-up clutch is fully engaged, the hydraulic torque converter does not have the ability to absorb the torsional vibrations that are created by the engine and the fracking pump. Torsional vibrations are known to spike in this fully locked condition, especially if a critical harmonic frequency exists in the operating range. Excessive torsional vibrations can reduce the life of various components in the powertrain system. An improved method to reduce these torsional vibrations was therefore needed.